The present invention relates to a method and apparatus for detecting transitions between different gas or liquid products in a flow path and, more particularly, it relates to an apparatus and method utilizing Raman spectroscopy for detecting transitions between petroleum products.
Before the advent of lasers, the use of Raman spectroscopy as a routine analytical technique was limited due to the small number of available sources of intense, monochromatic radiation Since the 1960xe2x80x2s, laser have become the excitation source of choice for Raman spectroscopy, as they provide much greater intensities and narrower line widths than the mercury arc lamps commonly employed previously. Furthermore, much weaker Raman signals became observable, resolution improved, which lead to laser Raman spectroscopy becoming an important benchtop tool for identifying molecular species via characteristic or xe2x80x9cfingerprintxe2x80x9d vibrational features. In the past, due to the special requirements of the lasers, (high voltages, cooling water, specialised personnel and space requirements), these systems tended to be large and expensive, and needed to be used in dedicated facilities.
Since then, diode lasers have become much simpler to operate. Diode lasers are small and inexpensive, can run off very low voltages (15 V or less), generate less heat, and have high conversion efficiencies compared to traditional laser Raman sources. In spite of these advantages, certain inherent properties of diode lasers have made them less appealing for use in Raman spectroscopy. These include lower intensities, a less monochromatic output (xe2x80x9cmode hoppingxe2x80x9d), greater beam divergence, and excitation wavelengths restricted to the near to mid-IR regions. With recent advances in diode laser technology, many of these difficulties have been overcome, and Raman spectrometers with diode lasers as the excitation source have begun to appear.
One big advantage of using diode lasers as a source for Raman spectroscopy was to move the technique out of the lab and into the realm of field measurements and process monitoring. Relatively high molecular weight organics, such as petroleum products, various plastics, and types of edible oils have been differentiated on the basis of their Raman spectra generated using a portable diode laser based Raman instrument. These compounds are good candidates for Raman based analysis due to the presence of strong Raman features with shifts in the region of 700-1700 cmxe2x88x921. Raman analysis of commercial grade gasoline in particular can benefit from an excitation source in the near to mid-IR, to avoid interference from background fluorescences which can be excited at lower wavelengths. These fluorescence signals depend on the excitation wavelength, and can be so strong that they completely obscure the Raman features which would otherwise appear.
A number of studies have used Raman spectroscopy to examine fuels or mock fuel mixtures. These include a quantitative analysis of xylene isomers in mock petroleum fuels using a diode laser Raman spectrometer with an excitation wavelength of 800 nm. A partial least squares regression analysis routine was used to correlate the individual xylene isomer concentrations to the Raman signal, without the use of an internal standard. For samples containing between 1.5 and 15% xylenes, the concentrations were determined to within xc2x10.1% for the meta- and para-isomers, and to within xc2x10.15% for ortho-xylene. Other studies include a comparison of near-IR and Raman spectroscopies for the determination of the chemical and physical properties of naptha, an analysis of aviation turbine fuel composition, and a system designed to correlate the Raman spectra of gasolines with their octane ratings. The first three studies were all laboratory based, while the fourth describes a partial least squares regression analysis routine which was applied to spectra recorded on a commercial FT-Raman spectrometer with a Nd:YAG source. A large xe2x80x9ctraining setxe2x80x9d of spectra taken from fuels with known octane ratings was used to build a model to predict the octane rating of fuels not included in the training set. The accuracy of the determined octane ratings depended on the accuracy of the training set used to create the model. In general, a given fuel octane rating can correspond to any number of different chemical xe2x80x9crecipesxe2x80x9d, i.e. the octane rating does not uniquely define the exact chemical composition of the fuel. Gasoline derived from a common source, or processed by a particular refinery, may show a particular pattern, which the training set can xe2x80x9clearnxe2x80x9d to recognize. However, a fuel derived from a different source may not match the defined pattern, necessitating the acquisition and use of a new training set.
The problem of measuring the octane rating of a given fuel, other than by the empirical xe2x80x9cengine knockxe2x80x9d tests used to define the quality, is clearly not straightforward. The training set approach does have its uses. However, it relies on being able to establish a reliable base (the training set), and can get somewhat cumbersome when applied to a large, widely varying set of samples. The problem of distinguishing between various grades of gasoline, without attempting to specify an octane rating, is significantly less demanding. It is also still extremely important. When the finished products leave the refinery, they travel through pipelines to distribution stations, where they are directed into holding tanks before being transported by truck to local filling stations. Any or all of the different grades of gasoline or distillates may pass sequentially through a given section of pipe. It is thus important to be able to determine exactly when one product lot ends, and the next begins.
Various techniques have been used to identify the exact product interface in such a setting. As a basic requirement, one needs a measurable physical or chemical property, which differs not only from product to product, but from grade to grade. Ideally, the measurements should be fast, non-destructive, able to be made in-situ, give a clearly visible (large) change when an interface occurs, and produce an output, which does not require a high degree of technical skill to interpret.
One technique is to measure the density of the products as they flow past a particular point near the outlet into the holding tanks. The density measured at this point in actually based on a sonometer, which measures the speed of sound as it passes through the sample. This technique produces large changes at gasoline to distillate interfaces, but can have trouble distinguishing between grades of gasoline, and between diesel vs. low sulphur diesel oils. Viscosity or colour changes have also been used. Another device, advertised commercially, detects interfaces by measuring the electrical resistivity of the product as it flows past a point.
The abundance of techniques available serves to demonstrate the importance of detecting product interfaces in a gasoline pipeline. Accordingly, there is a need for a method, which reliably and accurately detects product interfaces in a pipeline in real-time. Particularly, there is need for a method and apparatus which can accurately and reliably detect product interface between a wide range of petroleum products, such as gasoline products as well as other distillates (diesel, jet fuel, etc.).
In accordance with the present invention a compact, portable Raman spectrometer system suitable for in situ use in hostile environments has been proposed. The source is a high power (500 mW), broad band red diode laser which has been mode locked using an external cavity. This produces a monochromatic excitation beam at a wavelength of approximately 670 nm. The spectrometer consists of an entrance slit, a combined diffraction grating/focussing element, and an exit slit. The resolution of the Raman spectra obtained is excellent (7.2 cmxe2x88x921 FWHM at a Raman shift of 1000 cmxe2x88x921). The Raman signal, which exits the spectrometer exit slit is detected by a highly sensitive photomultiplier tube (PMT), and sent to a computer device (PC-104) for data acquisition and analysis.
The proposed invention described herein detects liquid or gas products in a flow path. Specifically, this invention detects changes in the composition of various petroleum products flowing through a gasoline pipeline, by means of exposing samples of various petroleum products to the Raman spectrometer system. The petroleum products of interest consisted of four distillates (diesel oil, low sulphur diesel, jet fuel, and furnace oil), and three different grades of gasoline (regular unleaded, premium 91, and premium US92). Both the hardware and software is tailored specifically to suit this application. A computer acquires full Raman spectra from the Raman spectrometer system, wherein data obtained from the spectra is processed using a multiple-dimension xe2x80x9cleast squaresxe2x80x9d routine.